The present invention relates to a process and a device for the continuous manufacture of a polymer of an aromatic alkylene such as styrene, especially comprising a step of recovering the unreacted monomer.
It is known practice to continuously manufacture aromatic alkylene polymers by a process of bulk polymerization, in particular a homopolystyrene or a copolymer of styrene having a high impact strength (“HIPS”) and which is especially modified by grafting onto a rubber. In such a process, the polymer is first continuously prepared by a bulk polymerization in the liquid monomer. This thus gives a mass of polymer melt, dispersed in the liquid monomer, which is then subjected to degassing, generally known by the term devolatilization. The aim of the devolatilization is to remove from the polymer the unreacted monomer and volatile compounds, such as oligomers formed especially during the polymerization and possibly hydrocarbon-based impurities accompanying the monomer, which are generally inert and have a low boiling point, such as, for example, ethylbenzene (EB), cumene, n-propylbenzene (NPB), methylcyclohexane (MCH) and ethyltoluene (ET).
It is known practice to perform the devolatilization in one or, preferably, at least two successive steps, under a pressure which is generally less than atmospheric pressure, as is described in American patents U.S. Pat. No. 3,853,672, U.S. Pat. No. 3,903 202 and U.S. Pat. No. 3,928,300 and in European patent application EP 0 323 428. The devolatilization gives one or more streams of gas, separated from the polymer, generally comprising the unreacted monomer and volatile compounds which may be recovered, for example, by condensation. The amount of monomer thus recovered is generally so large that it is necessary for economic reasons to recycle the monomer, for example into the polymerization.
In continuous process, it is thus understood that the recycling of the monomer inevitably results in an increase in the concentration of volatile compounds and especially of oligomers in the polymerization medium. In order to maintain the quality of the polymer produced at a high level, it has become necessary to monitor and limit the amount of oligomers returned into the polymerization, and it is therefore important to separate out and remove the oligomers from the monomer recovered during the devolatilization, before returning the monomer, for example, into the polymerization. A process has already been proposed for separating the oligomers from the monomer by means of a distillation performed according to a type of “delayed” operation (“off-line processing”). However, this process has many drawbacks, especially as regards the flexibility of a continuous process for manufacturing a polymer, the consistency of the quality of the polymer manufactured and the cost of the process.
American patent U.S. Pat. No. 3,886,049 describes a process for separating the oligomers from the monomer, on-line with a process for the continuous manufacture of an aromatic alkylene polymer comprising a devolatilization of the polymer in two successive steps. The gas stream resulting from the first step of the devolatilization feeds a first zone known as a “first contacting and separation or stripping zone” in which a gas phase relatively rich in monomer is separated from a liquid phase. The gas phase is recovered at the top of the first zone and is returned after condensation into the polymerization. The liquid phase which is relatively rich in oligomers is recovered at the bottom of the first zone and is then partly returned into the top of the first zone and partly removed via a purge or even returned into the polymerization. The gas stream resulting from the second step of the devolatilization feeds a second zone known as a “second contacting zone” in which the gas stream is placed in direct contact with a liquid stream derived from a portion of the liquid leaving at the bottom of this zone, after having been cooled beforehand and optionally supplemented with a fresh supply of liquid monomer. The second zone behaves essentially and solely as a direct-contact condenser, such that the gas stream feeding this zone is totally condensed and forms an entirely liquid phase. This entirely liquid phase leaves at the bottom of the second zone and is then cooled. A portion of the entirely liquid phase is then returned into the second zone, such that all of the gas stream feeding this zone is condensed. The other portion of the entirely liquid phase is sent into the first zone in which it is placed in direct contact with the gas stream resulting from the first step of the devolatilization.
It has been observed that the process described by American patent U.S. Pat. No. 3,886,049 does not make it possible to achieve a sufficiently effective separation between the oligomers and the monomer, and that the monomer thus separated out still contains relatively large amounts of oligomers, such that the polymer thus manufactured is of relatively mediocre quality.
A process has now been found for overcoming the difficulties mentioned above. In particular, a process has been found for the continuous manufacture of an aromatic alkylene polymer, comprising a devolatilization in at least two successive steps, and a method which is sufficiently effective for continuously separating the oligomers from the monomer in the gas streams resulting from the devolatilization, so as to be able to recover the monomer with a very low content of oligomers, for example a content of less than 1%, preferably less than 6.1% or even less than 0.01% by weight.